The present invention concerns a method for operating a drive-train with a hybrid drive system. Such a hybrid drive system comprises at least one electric machine and an internal combustion engine. In addition to the hybrid drive system a drive-train of a hybrid vehicle comprises a transmission, which converts rotational speeds and torques so as to transmit a traction force provided by the hybrid drive system to a drive output of the hybrid vehicle. Moreover, a drive-train of a hybrid vehicle comprises a brake system and it is already known from the prior art that a brake system can comprise more than one partial brake system, namely a so-termed service brake which acts by friction on wheels of the drive output, and at least one wear-free permanent brake. The wear-free permanent brake can be a so-termed engine brake or a retarder of the drive-train.
From DE 198 43 580 A1 a brake system of a motor vehicle is known, in which a required braking force or braking torque is distributed between the friction brake of the brake system and the permanent brake thereof. According to this prior art, it is proposed that when the permanent brake is actuated the friction brake is acted upon by a pressure, which at first corresponds to a signal from a braking value emitter and, as the braking force of the permanent brake increases, is reduced in such manner that the total braking force corresponds to the signal from the braking value emitter. In this way the friction brake is fully effective in situations requiring a short response time, whereas during a longer braking operation the permanent brake takes over all of the braking action and so protects the friction brake from overload.
US2005/255965 A1 and EP 2 127 987 A1 described methods such that in a hybrid drive system with an internal combustion engine and an electrical drive unit, the braking torque is initially applied by the electrical drive unit and thereafter, as a function of the characteristics of the permanent brake, is distributed between the electrical drive unit or electric brake and other permanent brake in such a manner that the sum corresponds to a braking requirement.